Shock absorbing devices of cylindrical construction which are used for protection, find applications in a wide variety of situations. At one extreme of size are marine fenders which are employed to minimize the possibility of damage to wharves and ships during docking procedures, or in heavy seas. At the other extreme of size are shock absorbing components utilised in machines and instrumentation. The aim of this invention is to produce a fender having stable operative characteristics and large energy absorbing ability, relative to its mass and size, coupled with a maximum reaction force when compressed over its designed deflection characteristics which do not exceed the strength of the surfaces or members being protected.
The use of marine fenders to protect ships, wharves, drilling rigs and similar marine structures is well known. Typically these are of substantially cylindrical or tubular construction and may be of circular, D, trapezoid or rectangular cross-section. Various other designs have been employed including inflatable fenders and floating fenders.
Typically tubular fenders are comprised of rubber material or in particular styrene butadiene rubber (SBR). In addition some fenders are formed with metal or hard plastic sections or inserts to provide additional durability, toughness and means of mounting.
Tubular fenders are usually designed to absorb energy by axial or radial elastic compression. The majority of fenders loaded axially are contained within or attached to complex rigid structures or have sophisticated mounting requirements and shapes to handle large deflections which are desirable to minimise the reaction force, which becomes critical when cushioning larger vessels especially those above 150,000 tons, as their steel plate thickness does not increase in direct proportion to their mass. Thus their cost effectiveness becomes less with increasing size. In addition any of these mounting structures which have a considerable inertial mass which is added to the inertial mass of the rubber fender further increases the reaction force to a degree where the vessel's hull is damaged especially where the closing velocity is high.
Similarly prior art tubular fenders which are compressed radially and having other than a substantially circular cross section also contain complexities which reduce their cost effectiveness. Furthermore as these more complex shapes need to be molded as monolithic rubber members for maximum effectiveness and durability there is a practical limitation to their unit size and mass dictated by technological and tooling cost considerations.
Conversely currently used rubber tubular fenders which are compressed radially and having substantially circular cross-sections may be manufactured by a process whereby a strip of uncured rubber is wound around a mandrel until the desired diameter is reached. This lamination is then contained, and cured with heat and pressure. This allows for the manufacture of very large fenders weighing up to 15 tons and costing tens of thousands of dollars. And although their energy absorption per unit mass may not be as efficient as smaller more complex shapes their relatively lower manufacturing maintenance and mounting costs sees their increasing use, even in smaller sizes, typically of 0.4 m O.D. and 0.2 m I.D. where they are installed in lengths secured to docksides or vessels by wires or chains threaded through their hollow cores. Even so it will be appreciated that the larger items are expensive and both labour and material intensive to produce and difficult to handle.
In addition the above fenders commonly of substantially cylindrical construction with a hollow core may be supported by a member or members passed through the hollow core and each end is attached to the marine structure. The fender is thus slung against the side of the structure. A common support member is a semi-elliptical metal rod supported by chains.
These fenders have different operative characteristics depending on the degree of compressive load to which they are subjected. For low loads, the amount of energy absorbed may be a linear function of the radial deflection of the fender surface and the Shear Modulus (G) of the rubber which is dependant on the IRHD of the rubber. For thin sections the load-deformation behaviour has been derived by considering the bending moment that exists at any cross-section. For thick sections the shearing forces and normal forces must be considered. In use the hollow core of the fender may be flattened. At the point inner surface defining a hollow core has been totally compressed the energy absorbing characteristics change to those of a solid pad under compression and the IRHD of the bulk material determines the reaction force. Thus for this type of fender the best performance is achieved where the designed fenders absorb the energy of impact before reaching the limit wherein the characteristics are those of the bulk material.
The publication titled "Theory and Practice of Engineering with Rubber" which has a Library Congress catalogue card number 78-325872 gives a comprehensive outline of rubber design and calculation principles and specifically pages 146 to 165.
With respect to the radial compression of long hollow substantially circular cylinders where the ratio of the outside diameter (O.D.) divided by the internal diameter (I.D.) is generally less than 2.5, and referring to the above publication page 148 onwards and applying general engineering principles with respect to bending stresses in curved beams, it can be appreciated that the maximum fiber stress due to bending moments occurs at the diametric plane normal to the applied force. Provided that the cross-section is regular, and the Shear Modulus is essentially constant over the curved section then the fiber stress varies from a maximum compressive stress at the internal surface to zero at the neutral axis to a maximum tensile stress at the external surface. For this situation the stress distribution is of a hyperbolic nature and the neutral axis is located at a radius other than the radius of the centroid axis. In this situation the neutral axis is located between the centroid axis and the center of curvature; this always occurs in regular sectioned beams of constant material strength. Of course this may not be the case if the sectional area or material strength varies in a radial direction.
Furthermore it can be shown by calculation that for a symmetrical section the maximum bending stress will always occur at the inside fiber surface of the fender. Calculations show that for a fender of O.D. divided by I.D.=2 this bending stress at the inner surface is approximately 25% higher than that at the outer surface.
It is also evident that for a constant reaction force supplied by a fender the discussed bending stresses applied to the fender fibers increase as the ratio of the O.D. divided by the I.D. decreases. For example decreasing the ratio from 2 to 1.5 doubles the bending stresses.
It is an object of this invention to provide a shock absorbing marine fender which provides protection against damage due to impact, and which is simpler to produce and easier to handle than existing devices. It is a further object to provide shock absorbing devices of this invention to have higher lead bearing capacity per unit weight (compared with existing products), and resistance to tear and cut propagation, than existing products.
In this invention a suitable shock absorbing fender can be produced from polyurethane elastomer material. Such fenders perform at least as well as rubber or SBR equivalents and in many aspects are far superior.